1. Field of the Invention
The present invention relates to a pulse-wave-propagation-velocity measuring apparatus which measures a velocity at which a pulse wave propagates in a living subject.
2. Related Art Statement
Pulse-wave propagation velocity is utilized in various medical diagnoses. For example, since pulse-wave propagation velocity increases as artery hardens, the velocity is used in diagnosing arteriosclerosis. In addition, since pulse-wave propagation velocity changes in relation with changing of blood pressure, the velocity is used in monitoring the blood pressure.
A pulse-wave propagation velocity may be measured such that two heartbeat-synchronous signals (a pulse wave, a heart sound, an electrocardiograph signal, etc.) are detected from two portions of a living subject, a propagation time in which one of the two signals propagates between the two portions, is determined based on respective times when the two signals are detected, and the velocity is determined by dividing, by the propagation time, a length of an artery between the two portions.
In a conventional manner, a length of an artery is approximated by a value which is obtained by substituting a stature of a patient for an expression representing a relationship between stature and artery""s length that is experimentally determined in advance, or a value which is measured, on a body surface, between the two portions.
Thus, in order to measure an accurate pulse-wave propagation velocity, it is needed to measure an accurate artery""s length. However, the artery""s length determined based on the patient""s stature is not so accurate. In addition, the value measured on the body surface between the two portions is more accurate than the artery""s length determined based on the patient""s stature, but the accuracy of measurement depends on measurer""s skill. In addition, the measuring operation needs a long time.
It is therefore an object of the present invention to provide a pulse-wave-propagation-velocity measuring apparatus which can accurately and quickly determine a length of an artery of a living subject.
The above object has been achieved by the present invention. According to a first aspect of the present invention, there is provided an apparatus for measuring a velocity at which a pulse wave propagates between a first portion and a second portion of a living subject, the apparatus comprising: a propagation-signal producing device which is adapted to be worn on the first portion of the subject and produces a propagation signal; a propagation-signal detecting device which is adapted to be worn on the second portion of the subject and detects the propagation signal that has been produced by the propagation-signal producing device, has propagated through a body tissue of the subject, and has reached the second portion; a blood-vessel-length determining means for determining a length of a blood vessel of the subject that is located between the first and second portions of the subject, based on a signal propagation time from a time when the propagation signal is produced by the propagation-signal producing device, to a time when the propagation signal is detected by the propagation-signal detecting device, and a pre-stored velocity at which the propagation signal propagates through a body tissue of a human person; and a pulse-wave-propagation-velocity determining means for determining the velocity at which the pulse wave propagates between the first and second portions of the subject, based on the length of the blood vessel determined by the blood-vessel-length determining means.
According to this aspect, the blood-vessel-length determining means determines, as a length of a blood vessel, a distance by which the propagation signal has propagated between the first and second portions of the subject where a pulse-wave propagation velocity is to be measured. Thus, the present apparatus can measure length of blood vessel with the same degree of accuracy as that with which length of blood vessel is measured on body surface between the two portions. In addition, since blood-vessel length can be determined by just wearing the propagation-signal producing device on the first portion and wearing the propagation-signal detecting device on the second portion, no skill is needed. Moreover, the time needed for the determination of blood-vessel length can be shortened.
Preferably, the measuring apparatus further comprises a first heartbeat-synchronous-signal detecting device which is adapted to be worn on the first portion of the subject and detects a first heartbeat-synchronous signal from the first portion; and a second heartbeat-synchronous-signal detecting device which is adapted to be worn on the second portion of the subject and detects a second heartbeat-synchronous signal from the second portion, and the pulse-wave-propagation-velocity determining means determines the velocity at which the pulse wave propagates between the first and second portions of the subject, by dividing the length of the blood vessel determined by the blood-vessel-length determining means, by a pulse-wave propagation time from a time when the first heartbeat-synchronous signal is detected by the first heartbeat-synchronous-signal detecting device, to a time when the second heartbeat-synchronous signal is detected by the second heartbeat-synchronous-signal detecting device. In addition, the propagation-signal producing device is supported by the first heartbeat-synchronous-signal detecting device, and the propagation-signal detecting device is supported by the second heartbeat-synchronous-signal detecting device.
Thus, when the first and second heartbeat-synchronous-signal detecting devices are worn on the subject, the propagation-signal producing and detecting devices are also worn on the subject. Thus, the pulse-wave-propagation-velocity measuring operation can be easily done.
However, it is possible to employ a propagation-signal producing device which is separate from the first heartbeat-synchronous-signal detecting device, and a propagation-signal detecting device which is separate from the second heartbeat-synchronous-signal detecting device. In this case, the apparatus can employ, as the first and second heartbeat-synchronous-signal detecting devices, conventional heartbeat-synchronous-signal detecting devices as they are.
According to a second aspect of the present invention, there is provided an apparatus for measuring a velocity at which a pulse wave propagates between a first portion and a second portion of a living subject, the apparatus comprising a bed on which the subject lies; a contour determining device which determines a contour of the subject lying on the bed; a blood-vessel-length determining means for determining a length of a blood vessel of the subject that is located between the first and second portions of the subject, based on the contour of the subject determined by the contour determining device and respective positions of the first and second portions of the subject in the contour; and a pulse-wave-propagation-velocity determining means for determining the velocity at which the pulse wave propagates between the first and second portions of the subject, based on the length of the blood vessel determined by the blood-vessel-length determining means.
According to this aspect, the contour determining device determines the contour of the subject whose pulse-wave propagation velocity is to be measured, and the blood-vessel-length determining means determines the length of the blood vessel based on the contour of the subject determined by the contour determining device and the respective positions of the first and second portions in the contour. Thus, the present apparatus can determine the length of the blood vessel with the same degree of accuracy as that with which a distance between the two portions on a body surface is measured. In addition, no skill is needed to determine blood-vessel length, and the time needed for the determination can be shortened.
According to a third aspect of the present invention, there is provided an apparatus for measuring a velocity at which a pulse wave propagates between a first portion and a second portion of a living subject, the apparatus comprising: a blood-vessel-image taking device which takes an image of a blood vessel of the subject; a blood-vessel-length determining means for determining a length of a portion of the blood vessel of the subject that is located between the first and second portions of the subject, based on the image of the blood vessel of the subject taken by the blood-vessel-image taking device; and a pulse-wave-propagation-velocity determining means for determining the velocity at which the pulse wave propagates between the first and second portions of the subject, based on the length of the portion of the blood vessel determined by the blood-vessel-length determining means.
According to this aspect, the blood-vessel-image taking device takes the image of the blood vessel and the blood-vessel-length determining means determines the length of the blood vessel based on the image of the blood vessel of the subject taken by the blood-vessel-image taking device. Thus, the present apparatus can determine the length of the blood vessel with a much higher accuracy than an accuracy with which a distance between the two portions on a body surface is actually measured.